WordWeb Online

1. Causing fear or anxiety by threatening great harm
   "grievous bodily harm";
   - dangerous, grave, serious, severe, life-threatening
    
2. Causing or marked by grief or anguish
   "a grievous loss";
   - heartbreaking, heartrending
    
3. Of great gravity or crucial import; requiring serious thought
   "a grievous fault";
   - grave, heavy, weighty
    
4. Shockingly brutal or cruel
   "a grievous offence against morality";
   - atrocious, flagitious, monstrous

See also: critical, evil, important, of import, sorrowful

Encyclopedia: Grievous